1. Field of the Invention
The present invention relates to a camera lens assembly and more specifically to a camera lens assembly which includes a drive motor arrangement therein.
2. Description of the Background Art
In order to facilitate a better understanding of the invention, a typical construction of a camera lens assembly and a lens drive mechanism in the prior art will be discussed herebelow with reference to FIG. 1.
FIG. 1 is a sectional elevation showing the camera assembly and lens drive mechanism in the conventional art.
FIG. 1 shows a camera lens assembly for use in video cameras and the like which are equipped with an auto-focus system and which includes a PM type stepping motor. This system was disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-151606, published on Aug. 9, 1985, for example. As shown, this arrangement includes a movable front lens tube section 1, a fixed lens tube section 2 and an inner rotor type lens drive PM stepping motor 3 which is defined therebetween. Viz., in this arrangement the front lens tube section 1 includes inner and outer coaxially arranged, axially extending and concentrically arranged cylindrical portions 1a and 1b. The outer cylindrical portion 1a is provided to permit manual focussing (manipulation) of the lens assembly 8 while the inner portion 1b is provided with a rotor magnet 4 on its outer periphery. The rotor magnet 4 and the inner section 1b form the rotor of the stepping motor 3. The fixed lens tube section 2 also includes inner and outer coaxially extending and concentrically arranged portions 2a and 2b. The inner portions 1b and 2b are operatively interconnected by a helicoid gear 5 formed therebetween. The outer section 2b of the fixed lens tube section 2 supports two axially arranged coils 6, 6 which define part of a plural pole yoke member 7 of the stepping motor 3. When the stepping motor 3 is energized, rotation of the rotor (1b,4) induces the focussing lens 8 to move along its focal axis.
However, this arrangement has suffered from the drawbacks that it is necessary to provide a clearance between the rotor and the yoke 7 and between the inner periphery of outer portion 1a and the outer periphery of portion 1b to permit the required mutual rotation therebetween. This, in combination with the provision of the rotor magnet 4 on the outer periphery of the inner portion 1b causes an increase in the diameter of the yoke 7 which in turn increases the diameter of the unit as a whole. Further, due to the provision of the helicoid gear 5 between the inner periphery of the inner portion 1b and the outer periphery of the inner portion 2b, the length of the movable first section 1 of the lens tube is undesirably increased.
In addition to these problems, the provision of the rotor magnet 4 in the illustrated position causes the diameter to be smaller than stator yoke 7 and therefore reduces the maximum amount of torque which can be applied to rotate the front lens tube section 1 and thus move the lens 8. Moreover, the rotor magnet 4 is made of a rubber magnetic material, and due to the elastomeric nature of this material, it is difficult to ensure that this element is precisely secured in place. This tends to degrade production efficiency.